I am sure you can buy a used one much cheaper than building one from new parts. The blades and spool spacers are pretty expensive. I just looked at Agrisupply and bearing spools are $23, bearing spacer, $13.50, rear bumper end washer $14, then bearing are $20 and you need to build bearing carrier out of 1/2" plate, need axles, heavy steel 2x4 tubing for the frame and each 20" diameter blade runs $15-20 each. A 6 foot disk would have 4 blades to the gang for a total of $320 just for the blades. You can buy a nice disk used for $500.